What does a CNN classifier default to?
I am assuming the images you gave the model all contain sheep. This is what i understand from your question.

Any model that you build will be based on the data that you give (training data) and your code. In your case, if you only give images that contains sheeps, and then you test it with no sheep and a background the model hasn't seen, it will search through all of its node, derived from your training data, to see which one is closest to the image you gave. Based on the information given, the only 'route' your cnn model can take is the one with sheeps on it, because you only gave images of sheeps for it to learn.

Here are a few suggestions that i can give you:


Give your model images with no sheep and different backgrounds so it can handle cases where there are no sheeps
Or you can add a piece of code to your model that tells it to default to some value if certain conditions aren't met (If the model doesn't 'see enough' sheep and background, it defaults to 'unknown image' or whatever you choose)


You are on the right path though! Since you are playing around with a binary classifier, you should definitely feed images with and without sheeps so that it can identify the two cases. Remember, a binary classifier is optimal when you give it 2 things to look out for, such as identifying images with and without sheep.

Here are some reading materials that you can brush up on to get a better basic understanding of how CNN works, personally i found the video helpful on the second link:


https://becominghuman.ai/what-exactly-does-cnn-see-4d436d8e6e52
https://brohrer.github.io/how_convolutional_neural_networks_work.html


Also, i find google images helps me alot in terms of visualizing binary classifier as a concept.